1. Field of the Invention
The present application relates generally to systems and methods for determining the position of a directional drilling tool using measurement while drilling.
2. Description of the Related Art
Directional drilling for the exploration of oil and gas deposits advantageously provides the capability of generating boreholes which deviate significantly relative to the vertical direction (that is, perpendicular to the Earth's surface) by various angles and extents. In certain circumstances, directional drilling is used to provide a borehole which avoids faults or other subterranean structures (e.g., salt dome structures). Directional drilling is also used to extend the yield of previously-drilled wells by reentering and milling through the side of the previously-drilled well, and drilling a new borehole directed so as to follow the hydrocarbon-producing formation. Directional drilling can also be used to provide numerous boreholes beginning from a common region, each with a shallow vertical portion, an angled portion extending away from the common region, and a termination portion which can be vertical. This use of directional drilling is especially useful for offshore drilling, where the boreholes are drilled from the common region of a centrally positioned drilling platform.
Directional drilling is also used in the context of horizontal directional drilling (“HDD”) in which a pathway is drilled for utility lines for water, electricity, gas, telephone, and cable conduits. Exemplary HDD systems are described by Alft et al. in U.S. Pat. Nos. 6,315,062 and 6,484,818. Such HDD systems typically drill along relatively short distances substantially horizontal to the surface and do not drill very far below the surface.
The pathway of a directionally drilled borehole is typically carefully planned prior to drilling, and the position and direction of the drilling tool is repeatedly determined during the drilling process using surveys to map the pathway relative to a fixed set of known coordinates. In wireline surveys, the drilling of the borehole is periodically halted and a survey tool is lowered into the borehole. In some instances, the drilling assembly (i.e., the drilling tool and the drill string) is removed from the borehole so that the survey tool can be lowered into the borehole, while in other instances, the survey tool is inserted into the drilling assembly itself. As the survey tool is guided along the borehole, it provides information regarding its orientation and location by sending signals through a cable to the surface. This information is then used to determine the pathway of the borehole. The survey tool is then removed from the borehole and, in instances in which the drilling assembly had been removed from the borehole, the drilling assembly is returned to the borehole to continue drilling. Such wireline surveys thus require extensive time and effort to repeatably stop drilling, insert the survey tool into the borehole, and remove the survey tool from the borehole. Since the costs associated with operation of a drilling system can be quite high, any time reductions in borehole surveying can result in substantial cost savings.
In so-called “measurement while drilling” (“MWD”) surveys, the MWD survey tool is a component of the drilling assembly, typically in proximity to the drilling tool, and it remains within the borehole throughout the drilling process. MWD survey measurements of the orientation and location of the MWD survey tool are made without removing the drilling assembly from the borehole. Typically, MWD survey measurements are taken during periods in which additional drill pipes are connected to extend the drill string and the drilling assembly is substantially stationary, which takes approximately one to two minutes to a few minutes. Use of MWD surveys saves time during operation of the drilling system by eliminating the need to remove and replace the survey tool in order to survey the pathway of the borehole.